1. Field of Invention
The present invention relates to a light guide plate, surface light source device and liquid crystal display, in particular to improvements of a light guide plate having a back face provided with a great number of micro-reflectors, a surface light source device employing an improved light guide plate and a liquid crystal display employing the surface light source device for illumination a liquid crystal display panel. The present invention is applied to liquid crystal displays for portable telephones, personal computers or car-navigation systems, being applied further to surface light source devices and light guide plates used therein.
2. Related Art
According to a well-known prior art, a surface light source device has a light guide plate which is supplied with light sideways and outputs the light through an emission face after introducing the light into the light guide plate and applying direction-conversion, being broadly employed for illuminating an LCD panel or other uses. Although rod-like fluorescent lamps (cold cathode tubes) have been used broadly as primary light sources, those using point-like light sources such as LEDs (Light Emitting Diodes) tend to be employed recently.
In such surface light source devices, light guide plates are in charge of light-direction-conversion because the light guide plates are supplied light sideways. As known well, light-direction-conversion within light guide plates and promotion of emission from an emission face can be performed by employing light guide plates made of light scattering-guiding material, or by applying emission promoting processing such as making a back face or emission face light-diffusible.
However, as known well, such means causes the emitted light to be preferentially directed to a much inclined forward (Oblique emission directivity of light guide plate). Such greatly inclined output directions are much quite different from usually desired output directions.
According to a prior proposition to realize a direction-conversion capable of providing a preferential output direction which is desired, a great number of micro-reflectors are formed on a back face of a light guide plate. The micro-reflectors on the back face of the light guide plate proposed are shaped like a great number of micro-projections or micro-dents, which generate an inner propagation light proceeding toward an emission face by means of an inner-face reflection of them. This inner propagation light is emitted from the emission face, becoming an output light. The preferential output direction is controllable in a remarkable range within which a frontal direction falls, through design of shapes or postures of the micro-reflectors. FIGS. 1a, 1b, 2a and 2b show an example in which a surface light source device employing a light guide plate provided with micro-reflectors as mentioned above for illumination a liquid crystal display panel.
FIG. 1a is a plan view of an outlined arrangement viewing from the back side of the light guide plate, and FIG. 1b is a side view from the left side in FIG. 1a. 
FIGS. 2a and 2b illustrate an arrangement of micro-reflector 20 in a prior art, FIG. 2a showing in and around circled portion A, FIG. 2b showing in and around circled portion B.
In the first place, referring to FIGS. 1a and 1b, an incidence face 12 is provided by a side end face of a light guide plate 10 made of a transparent material such as acrylic resin or polycarbonate (PC). A rod-like primary light source (cold cathode tube) L1 is disposed along the incidence face 12 which is supplied with light from the primary light source. The light guide plate 10 has major faces 213 and 14 one of which provides an emission face 13. The other face (back face) 14 is provided with a great number of micro-reflectors 20 shaped like micro-projections.
A well-known liquid crystal display panel PL is disposed on the outside of the emission face 13 to provide a liquid crystal display of backlighting type. It is noted that size values are merely examples, being indicated in mm.
The primary light source L1 emits light, which is introduced into the light guide plate 10 through the incidence face 12. An inner propagation light travels within the light guide plate 10 and undergoes direction-conversion when entering into micro-reflectors 20 on being inner-reflected by inner faces of projections, with the result that light proceeding toward the emission face 13 is produced. Such inner reflection occurs once or twice depending on configuration of the micro-reflectors 20 (See concrete examples in embodiments described later).
The example of arrangement of micro-reflectors 20 shown in FIGS. 2a and 2b is determined as to meet the following conditions.
(1) Micro-reflectors 20 are arrayed with sidewise intervals on many imaginary lines drawn approximately in parallel with the incidence face 12. It is noted that the term xe2x80x9cimaginary linexe2x80x9d in the instant specification means line that is imaginatively drawn to determine or describe micro-reflector arrangement.
In FIGS. 2a and 2b, the imaginary lines are represented by the i-th, i+1-th, j-th and j+1-th imaginary lines in order from the side of the incidence face 12, Gi, Gi+1, Gj and Gj+1.
(2) Longitudinal intervals between imaginary lines Gi and Gi+1 (i=1, 2, 3 . . . j, j+1 . . . in the same manner hereafter) adjacent to each other get narrower according to an increasing distance from the incidence face 12.
In the illustrated example, 79 xcexcm and 58 xcexcm is set in and around circled portion A (near to the incidence face 12) and circled portion B (far from the incidence face 12), respectively (See FIGS. 1a, 1b, 2a and 2b).
(3) Sidewise intervals of many micro-reflectors 20 on each imaginary line Gi are constant. This is defined as di.
(4) Mutual sideway interval di on imaginary line Gi gets smaller according to an increasing distance from the incidence face 12. In the illustrated example, 79 xcexcm is set in the vicinity as shown in FIG. 2a (circled portion A) of the incidence face 12 and 58 xcexcm is set in a distant portion (circled portion B as shown in FIG. 2b far from the incidence face 12. In other words, sideway interval is equal to longitudinal interval between successive imaginary lines at each portion correspondingly.
As a natural result derived from these conditions (1) to (4), micro-reflectors 20 are arranged as to get denser according to an increasing distance from the incidence face 12 regarding both directions, sidewise direction (parallel with imaginary line Gi) and longitudinal direction (perpendicular to imaginary line Gi). Such variations in arraying density (more generally expressing, covering rate) of micro-reflectors enables the emission face 13 to provide a uniformalized brightness (emission face intensity).
Cases where a great number of micro-reflectors are formed on a back face of a light guide plate as described above are subject to a problem that the micro-reflector arrangement is apt to be visible through the light guide plate. A sample prepared according to the above example of arrangement (FIGS. 2a and 2b) was found fairly visible.
This problem is relaxed to some degree by reducing size of individual micro-reflectors. However, size reduction is subject to a practical limit. According to another technique, a light diffusion plate having a strong diffusibility is disposed on an emission face, which brings loss of light. Beside, it is not preferable that an arrangement of light diffusion late is indispensable, viewing from
According to another idea, it is attempted that micro-reflectors are arranged as randomly as possible in order to avoid the arranged micro-reflectors from being conspicuous. However, if this idea is applied, local pattern rather appears, which is probably caused by xe2x80x9cfluctuation of arranging ratexe2x80x9d, failing to provide good results.
Thus an object of the present invention is to provide a light guide plate improved so that the above-mentioned micro-reflector arrangement is inconspicuous, and to provide a surface light source device improved by employing the light guide plate and a liquid crystal display improved by employing the surface light source device.
According to the present invention, an inconspicuous micro-reflector arrangement is obtained by requiring micro-reflectors formed on a back face of a light guide plate to meet a certain conditions. That is, the present invention provides a light guide plate having a back face provided with an inconspicuous micro-reflector arrangement, a surface light source device employing the light guide plate and a liquid crystal display employing the surface light source device.
In the first place, the present invention is applied to a light guide plate supplied with light from a primary light source, comprising an emission face for light-outputting, a back face opposite with the emission face and an incidence face for light-inputting.
The back face is provided with a great number of micro-reflectors for light-proceeding-direction-conversion, which are arranged as to meet the following conditions.
Condition 1; The great number of micro-reflectors are arranged with sidewise intervals on many imaginary lines K1, K2, K3 . . . which are separated with longitudinal intervals to each other and extend in directions generally perpendicular with respect to an imaginary reference line S that extends as to leave the incidence face.
Condition 2; Covering rate of the micro-reflectors on the back face per unit area tends to increase according to an increasing distance from the imaginary reference line S.
Condition 3; When many micro-reflector trios are imaginatively composed of micro-reflectors arranged on successive three imaginary lines Ki, Ki+1, Ki+2 (i=1, 2, 3, 4 . . . ) so that each micro-reflector trio consists of three micro-reflectors which are located adjacent to each other and picked up one by one from every imaginary line Ki, Ki+1, Ki+2 and further so that each of approximately all of the micro-reflectors belong to only three trios of the many trios, the three micro-reflectors belonging to each of the many micro-reflector trios are located approximately on a straight line.
Condition 4; Covering rate of the micro-reflectors on the back face per unit area tends to increase according to an increasing distance from the incidence face.
Condition 2, which is important particularly, enables the micro-reflector arrangement to be inconspicuous in combination with the other Conditions 1 and 3. That is, the above-mentioned prior art example does not meet Conditions 2 while meeting Conditions 1, 3 and 4.
In other words, the above-mentioned prior art example (FIGS. 2a and 2b) shows that many micro-reflectors 20 are arranged with a constant mutual sidewise interval di on each imaginary line Gi.
As a result, even if an arrangement of micro-reflectors 20 on imaginary lines Gi and Gi+1 adjacent to each other are designed as to not align longitudinally, it is inevitable that oblique aligning lines, which extend approximately in parallel at a xe2x80x9cconstant sidewise intervalxe2x80x9d (for example, sidewise interval between Hk and Hk+1=sidewise interval between Hk+1 and Hk+2) with each other in a considerable length range, are produced in a wide area (generally across the whole width of the light guide plate).
It is noted that covering rate does not vary along a direction of toe width of the light guide plate because the micro-reflectors 20 are the same in size.
According to researches by the instant inventor, if such xe2x80x9cregularity of repeated arrangement of equal-sized micro-reflectors with sidewise intervalxe2x80x9d exists in a large area, conspicuous stripe-like pattern is apt to appear.
To the contrary, according to the present invention, although xe2x80x9coblique aligning linesxe2x80x9d are produced, sidewise intervals of the xe2x80x9coblique aligning linesxe2x80x9d vary depending on an increasing sidewise distance from an imaginary standard line by requiring Condition 2, as described later (See FIG. 4 described later). Alternately, covering rate may be varied by increasing micro-reflector size according to an increasing sidewise distance from an imaginary standard line.
In such ways, xe2x80x9cregularity of repeated arrangement of equal-sized micro-reflectors with sidewise intervalxe2x80x9d is avoided from existing in a large area, with the result conspicuous stripe-like pattern is prevented from appearing.
It is noted that configurations of micro-reflectors are not limited in particular so far as their functions are maintained, in other words, so far as an inner propagation light proceeding toward an emission face is produced by inner reflection.
Typical configurations of micro-reflectors include quadrangle pyramids, cylindrical dents and V-shaped dents (laid triangular poles). Concrete examples of configurations and arrangements of micro-reflectors employed in the present invention are described in embodiments later.
According to a preferable embodiment, an incidence face is set at a corner portion of a light guide plate. Light supply from a corner portion avoid any corner portion from receiving uniquely weak light, being highly suitable for micro-reflector arrangements as employed in the present invention.
The present invention is applied to a surface light source device comprising a primary light source and a light guide plate which introduces light through an incidence face to emit light through an emission face, wherein the light guide plate is one improved as above. The employed light guide plate provided with an inconspicuous micro-reflector arrangement causes the surface light source device to provide an improved light output quality such that unnatural bright-dark pattern is hardly visible.
In addition, a liquid crystal display showing an improved display quality is provided if the same surface light source device is employed for illuminating a liquid crystal display panel.